This map shows Professor Peter Hall's (head of the Bartlett School of Planning at UCL) plan for a London RER in the 1988 book "London 2001".
Ignore the Jubilee Line (obviously, as it was built differently, but there is a set of points and a stub at North Greenwich which could enable Dartford Loop services to be operated through it, using suitable trains of course.
This is what is said in the book about the lines. Remember this was written in 1988.
Line 1, the Waterpool Line would hook
up BR's shorter-distance Windsor Line services (including the Hounslow
loop, Shepperton and Windsor lines, and the Kingston loop), via Richmond
and Putney, with the Waterloo and City Line. Via a new transition north-east
of Clapham Junction, trains would descend into tube via stations at Battersea,
Chelsea (King's Road), Sloane Square, Vicotria, and Millbank (Lambeth Bridge),
to Waterloo. Thence the Waterloo and City Line would be reconstructed -
as already planned by BR - with a new interchange station at Blackfriars.
From Bank the line would be extended to a sub-surface station under Liverpool
Thence it would divide. The main branch would take in all the stopping services of the Hackney Downs Line to Enfield Town, Cheshunt, and Chingford. (Direct services via the Lea Valley route to Broxbourne and beyond would continue to terminate at Liverpool Street.) The other branch would take in the old Broad Street Line to Dalston Junction, thence joining the North London link tracks eastward via Hackney central to Stratford.